1. Field of the Invention
The present invention relates to a light detecting device and a light detection method which employ a superconductor, and more particularly, to a light detecting device and a light detection method which employ a Josephson junction.
2. Description of the Related Art
Signal detecting devices which utilize a superconductor, particularly, light signal detecting devices which utilize a superconductor, have been known [Japanese Journal of Applied Physics vol. 23 L333 (1984). Such light signal detection devices (shown in FIG. 30) have a microbridge type Josephson junction 192 which is formed of an oxide superconductor BaPb.sub.0.7 Bi.sub.0.3 O.sub.3 (BPBO) thin film 191, and utilize changes in the critical current of the Josephson junction caused by the radiation illuminated on the junction by means of an optical fiber 190. In the above-described detection device, a light-receiving portion is made of BPBO 191 having a critical temperature of about 13 K. In other words, liquid helium or the like must be used to operate the detecting device. Furthermore, the characteristics of the detecting device are dependent on those of the Josephson junction.
In the above-described conventional detecting device, the optical characteristics (e.g., spectral characteristics) of the device are determined by the spectral characteristics of the superconductor employed, i.e., wavelengths of the light to be detected are limited by the spectral characteristics of the superconductor, and detection of signals over a wide wavelength range is difficult.
Furthermore, in a case where the above-described detecting device is operated as a recording device, a semiconductor memory, or a Josephson memory, since it is operated by means of an electrical signal, it is easily affected by electrical noises caused by the extension of wiring.
Furthermore, in a case where the above-described detecting device is used together with a large number of other detecting devices, as in an image sensor, it is difficult to compensate for variations in the characteristics of the individual detecting devices caused during the manufacture thereof.
Furthermore, since the area of the junction to which radiation is applied is limited, highly accurate positioning is required.
The above-described conventional detecting device requires light having an intensity high enough to cause sufficient change in the critical current in the Josephson junction, which is difficult, and thus has a deteriorated sensitivity.
Furthermore, the use of the same Josephson junction as a light-receiving portion and a light detecting portion causes variations in the characteristics of the device.
Japanese Patent Laid-Open No. 50486/1989 discloses a light switching device (shown in FIG. 31) in which a PN semiconductor light signal sensing portion 204 is provided on a Josephson junction 203 of a superconductor 202 provided on a substrate 201. However, in such a light switching device, since the semiconductor light signal sensing portion 204 is directly laid over the Josephson junction 203 as if it covers the entirety of the Josephson junction 203, the light signal detecting sensitivity of the Josephson junction 203 is deteriorated. Furthermore, since the area of the Josephson junction 203 is very small, the semiconductor light signal sensing portion 204 formed on the Josephson junction 203 is also small, making introduction of light into such a small area with a high degree of accuracy difficult.